The invention relates to a device for the monitored control of a stepping motor, comprising a control device for generating digital control signals for the stepping motor, each digital signal being associated with one or more discrete, stable positions of the stepping motor and generating an energization pattern of the stepping motor windings corresponding to this position, and an angle sensor coupled with the stepping motor for generating feedback signals.
Such stepping motor control can be used with the display of the position in a topographic map, wherein a light cross projector is moved along two axes below the map by means of stepping motors, such that the projected light cross points to the position. The number of the steps carried out by the stepping motors is used as a measure of the position of the light cross projector. During operation under rough environmental conditions it may happen that the stepping motor becomes temporarily blocked due to mechanical jamming, variations of friction, contamination or the like, whereby the number of the steps carried out is no longer identical with the number of steps commanded (loss of steps). During operation of the stepping motor with certain stepping frequencies resonances caused by the moment of inertia of the load may occur and thus also involve the risk of a loss of steps. As the indicating range is limited by stops, a device must be provided which switches the motor off upon reaching the stop and causes a corresponding alarm.
In order to avoid blocking of the drive means due to unexpected load variations, stepping motors having ample driving torques are used in prior art devices. Because of the ample dimensioning of the driving torques of the stepping motors commanded steps could be assumed to be carried out. Thereby, however, the dimensions of such an instrument become undesirably large. In practice, it is often necessary to accommodate the map display instrument with the light cross projector and the drive therefor within a small flat housing below the map. In the prior art arrangement the stops are detected by limit switches or similar sensors, which have to be adjusted very accurately, namely, accurately to half-a-step distance.
Furthermore, it is known to couple the stepping motor through a transmission to an angle sensor. The angle sensor permits monitoring of the position of the drive means within the indicating range. The stepping motor is actuated until the desired position has been reached (follow-up principle). Potentiometers, synchros and angular encoders are used as angle sensors. As the position of the drive means is known at any time, stops may be omitted.
In the prior art arrangements the resonance phenomena of the stepping motor are suppressed by mechanical damping (brakes, hydraulic damping) or by avoiding the critical drive frequencies. Mechanical damping elements are very expensive and susceptible to trouble and reduce the reliability of the drive mechanism. The follow-up principle with an angle sensor covering the whole operating travel of the stepping motor through a plurality of revolutions is very complex and expensive in construction.
It is the object of the invention to construct a device of the type defined in the beginning such that small and space-saving stepping motors may be used and a monitoring device constructed with simple means provided to monitor, at any time, whether the commanded steps have been carried out by the stepping motor.
In accordance with the invention this object is achieved in that only one well-defined signal of the angle sensor is associated with each stable position of the stepping motor and that an evaluation circuit adapted to compare the control signal and the angle sensor signal is provided, said circuit being adapted to monitor the carrying-out of each individual commanded step by the stepping motor.
Thus in accordance with the invention the carrying out of each individual step is monitored by an angle sensor which, however, does not represent the whole range of settings but permits only discrimination between the possible electrically determined positions of the stepping motor within one single revolution thereof. Monitoring is effected by comparing the control signal applied to the stepping motor and associated with a certain electric energization pattern of the stepping motor windings to the signal provided by the angle sensor and corresponding to the angular position of the stepping motor. Thus each individual step is monitored. Each command initiating a step of the stepping motor is given a "receipt" by the angle sensor signal. The angle sensor needs not to be able to discriminate between the steps through the whole range of settings of the stepping motor. Thereby the whole setup is considerably simplified.
The angle sensor may be arranged to generate digital output signals each of which is associated with one or more angle ranges, each angle range covering the angle between adjacent positions of the stepping motor.
The number of the electric switching states discernible by the energization patterns of the stepping motor may be equal to the number of the discernible digital output signals of the angle sensor.
The number of the discernible digital output signals of the angle sensor may be equal to the largest submultiple of the number of the stable positions of the stepping motor.
For example, the stepping motor may have eight stable positions which correspond to the angular positions of 0.degree., 45.degree., 90.degree., 135.degree., 180.degree., 225.degree., 270.degree. and 315.degree.. Thus the angle between adjacent stable positions of the stepping motor is 45.degree., and the angle sensor has eight angle ranges or sectors of 45.degree. with associated output signals. The stepping motor and the angle sensor may be adjusted relative to each other such that each stable position of the stepping motor is in the center of such an angle range. In each one of the angle ranges, the angle sensor supplies a digital output signal, which may be, for example OO within the angle range about the 0.degree. position and in the angle range about the 180.degree. position of the stepping motor, OL within the adjacent angle range about the 45.degree. position of the stepping motor and about the 225.degree. position of the stepping motor, LL within the angle ranges about the 90.degree. position and about the 270.degree. position of the stepping motor, and LO within the angle ranges about the 135.degree. position and about the 315.degree. position of the stepping motor. Thus the angle sensor provides one out of four discernible digital output signals. Correspondingly the stepping motor with its two windings is arranged to be switched into four electric states, discernible by the energization patterns of the stepping motor windings, by means of four different digital control signals, which may also be designated OO, OL, LL and LO, each two diametrically opposite stable positions of the stepping motor being obtained by identical energization patterns of the stepping motor windings. Thus the number of the discernible electric switching states of the stepping motor is equal to the number of the discernible digital output signals of the angle sensor, namely, four, in both instances. The number of the discernible digital output signals of the angle sensor is equal to the largest submultiple, namely, four, of the number of stable positions of the stepping motor, namely, eight.
The evaluation circuit may be arranged to generate an error signal, when after a predetermined maximum time, after the control signal has been applied, the signal from the angle sensor does not yet adopt the value associated with this control signal.
Thus, there is a wait of a given maximum time after the application of the control signal during which the stepping motor has to be set into the angular position corresponding to this control signal. If this is not the case, for example, because a spindle jams, a failure signal will be generated.
It may happen, however, that the stepping motor only swings through the commanded new position, whereby the angle sensor provides, for a short time, the signal corresponding to the commanded position, but that the stepping motor leaves this position at once during this oscillation. This can be taken into account in that the evaluation circuit determines a minimum time and includes testing means which responds to whether the signal of the angle sensor associated with a given control signal is supplied by the angle sensor during said determined minimum time, and the setting of the stepping motor to the angular position given by the control signal being assumed only after this condition has been fulfilled.
The control and monitoring of the stepping motor may be carried out by means of an appropriately programmed microprocessor.